Kotlin Classes and Objects

Declaring a Class

A class is declared with the class keyword followed by a name. By convention, class names start with an uppercase letter. The simplest class can be completely empty, but most classes declare properties and functions inside curly braces.

class Person

fun main() {
    val person = Person()
    println(person)
}

Properties

Properties are variables that belong to a class. They can be declared with val (read-only, like a final field) or var (mutable). Kotlin automatically generates getters for val properties and getters plus setters for var properties, so you rarely write boilerplate accessor methods yourself.

class Person {
    var name: String = ""
    var age: Int = 0
}

fun main() {
    val p = Person()
    p.name = "Maya"
    p.age = 29
    println("${p.name} is ${p.age} years old")
}

Creating Objects

An object is created by calling the class name as if it were a function - notice there is no 'new' keyword in Kotlin, unlike Java or C#. Each call to the class constructor produces a distinct object with its own copy of the properties.

• A class is a template; it exists once in your code
• An object is an instance of that template; you can create many
• Each object has independent state unless properties are shared via companion objects
• Objects are created without the 'new' keyword in Kotlin

class Counter {
    var count: Int = 0
    fun increment() { count++ }
}

fun main() {
    val a = Counter()
    val b = Counter()
    a.increment()
    a.increment()
    b.increment()
    println("a=${a.count}, b=${b.count}")
}

Member Access

You access an object's properties and functions using dot notation: object.property or object.function(). This is the same syntax used across most OOP languages, which makes Kotlin approachable if you've used Java, C#, or Swift before.

Next, you'll learn how constructors let you set property values right when an object is created, instead of assigning them one by one afterward.